As a former wildlife rehabilitator, I had occasion to work with many injured ducks and geese rescued from lakes in our county. Ducks are a reasonably hardy breed; injuries encountered were usually dog bites or due to entanglement with fishing line. Caring for them is fairly easy, and does not require a federal or state permit because they are considered a domestic species.
When you encounter a disabled duck, place it in a large box, to confine it from further injury, and to calm it. Remove the duck from where it was injured.
FIRST AID
Examine the duck. Stretch out the wings, feel along the wing for any lumps, or bleeding. Look at the feet, body, head and rectal area for any injury or bleeding. If there is a cut of more than 1/2 inch, bleeding that cannot be stopped by direct pressure, or an obvious break in a bone, take the duck to a veterinarian for treatment.
If the duck has become entangled in fish line, have someone hold the duck while you carefully snip and remove the line. If the duck has swallowed the line, there is probably a hook on the other end, so this requires veterinarian care, also. If the injuries are minor, cleanse any open wound with 50% Betadine 50% water mixture, applied with a cotton ball, then apply a little Neosporin to the wound. Bandaging depends on the wound itself, and its location. Clean and treat the wounds daily.
HOUSING
The duck will be in your care for several weeks, and depending on its injury, you have to provide certain accommodations so that it will be able to eat, sleep and eliminate, without much effort.
Keep the duck in the garage while it is recovering. You don't want it to come in contact with other animals, as it could be frightened and may injure itself even more. Provide it with a box with old, clean towels. Make sure you have cut down one side of the box for easy entrance and exit. Change the towels each day.
FOOD
Each day, provide a high quality duck chow in a large, non-breakable container, like an cake pan. Provide clean water each day, also. Food and water pans should be washed each day, prior to adding clean food and water. Little treats like fresh fruit or whole grain bread can be offered also.
CLEANLINESS
Newspaper can be spread over the entire garage floor prior to housing the duck. Ducks can be messy, and soiled newspapers should be refreshed with new, clean newspapers each day.
Within a week or two, your duck should be well on the way to a full recovery. Please have the duck evaluated by a veterinarian to determine when it can be released. Rehabilitating a duck can be very rewarding. The family can become involved, and children will learn compassion, caring and respect for life, while helping care for a disabled duck.